In the Fischer-Tropsch synthesis reaction a gaseous mixture of carbon monoxide and hydrogen is reacted in the presence of a catalyst to give a hydrocarbon mixture having a relatively broad molecular weight distribution. This product is predominantly straight chain, saturated hydrocarbons which typically have a chain length of more than 2 carbon atoms, for example, greater than 2 carbon atoms. The reaction is highly exothermic and therefore heat removal is one of the primary constraints of all Fischer-Tropsch processes. This has directed commercial processes away from fixed bed operation to slurry systems. Such slurry systems employ a suspension of catalyst particles in a liquid medium thereby allowing both the gross temperature control and the local temperature control (in the vicinity of individual catalyst particles) to be significantly improved compared with fixed bed operation.
Fischer-Tropsch processes are known which employ slurry bubble columns in which the catalyst is primarily distributed and suspended in the slurry by the energy imparted from the synthesis gas rising from the gas distribution means at the bottom of the slurry bubble column as described in, for example, U.S. Pat. No. 5,252,613.
The Fischer-Tropsch process may also be operated by passing a stream of the liquid medium through a catalyst bed to support and disperse the catalyst, as described in U.S. Pat. No. 5,776,988. In this approach the catalyst is more uniformly dispersed throughout the liquid medium allowing improvements in the operability and productivity of the process to be obtained.
We have recently found that a Fischer-Tropsch process may be operated by contacting synthesis gas with a suspension of catalyst in a liquid medium in a system comprising at least one high shear mixing zone and a reactor vessel. The suspension is passed through the high shear mixing zone(s) where synthesis gas is mixed with the suspension under conditions of high shear. The shearing forces exerted on the suspension in the high shear mixing zone(s) are sufficiently high that the synthesis gas is broken down into gas bubbles and/or irregularly shaped gas voids. Suspension having gas bubbles and/or irregularly shaped gas voids dispersed therein is discharged from the high shear mixing zone(s) into the reactor vessel where mixing is aided through the action of the gas bubbles and/or the irregularly shaped gas voids on the suspension. The suspension present in the reactor vessel is under such highly turbulent motion that any irregularly shaped gas voids are constantly coalescing and fragmenting on a millisecond time frame, for example, over a period of up to 500 ms. The transient nature of these irregularly shaped gas voids results in improved heat transfer and mass transfer of gas into the liquid phase of the suspension when compared with a conventional slurry bubble column reactor. The reactor vessel may be a tank reactor in which case a suspension recycle stream is withdrawn from the reactor vessel and may be recycled to the high shear mixing zone(s) via an external conduit. Exothermic heat of reaction may be removed from the system by means of a heat exchanger positioned in the external conduit (external heat exchanger) and optionally by a heat exchanger, for example, cooling tubes or coils positioned within the suspension in the reactor vessel (internal heat exchanger). This process is described in WO 0138269 (PCT patent application number GB 0004444) which is herein incorporated by reference. However, there may be a limit on the temperature to which the suspension may be cooled by the external heat exchanger owing to the risk of quenching the reaction and/or deactivating the catalyst. In the absence of an internal heat exchanger, this may necessitate circulating suspension around the external loop conduit at an uneconomic flow rate. The use of an internal heat exchanger may not entirely overcome this problem since there is a risk that an internal heat exchanger may interfere with the process fluid-dynamics, for example, causing coalescence of the gas bubbles and/or the irregularly shaped gas voids thereby reducing mass transfer of gas into the liquid phase of the suspension.